1. Field of the Invention
This invention relates to a method for producing pure water and, in particular, to a method to be used, for example, for manufacturing the equipment for producing ultrapure water used for rinsing semiconductor devices and the equipment for producing pure water for making up condensate at fossil-fueled and nuclear power stations, and for preparing pure and ultrapure water used in the pharmaceutical industry.
2. Description of the Related Art
In the manufacture of semiconductor devices, pure water refined to ultra-high purity, i.e., so-called ultrapure water, has conventionally been used for rinsing wafers, etc., which water contains very small quantities of electrolytes, particulates, bacteria or the like. To remove the small quantities of electrolytes, particulates, organic matter, bacteria, etc. contained in pure water, an apparatus for producing ultrapure water generally consists of a combination of systems using ion exchange resins, membrane separation means such as reverse osmosis membranes or ultrafiltration membranes, ultraviolet ray oxidation means, ion exchange polishers, etc. FIG. 9 shows a conventional example of such an apparatus for producing ultrapure water. In the apparatus shown, the suspended matter in raw water is removed by pre-treatment devices such as a coagulation and sedimentation unit, a filter or the like. Then, most of the electrolytes, particulates, bacteria, etc. are removed by a deionization device using ion exchange resins, a reverse osmosis device, a vacuum deaerator, an ion exchange mixed-bed ion exchange polisher, etc., which constitute a primary pure water producing system. Trace amounts of impurities remaining in the primary pure water, obtained by the primary pure water producing system, are removed by performing further sterilization and organic matter decomposition on the primary pure water with a secondary pure water producing system, which consists of an ultraviolet ray device (for sterilization and oxidation), an ion exchange polisher, an ultrafiltration membrane, etc. In many cases, the secondary pure water producing system is provided in the vicinity of use points distant from the primary pure water producing system.
In the field of semiconductor device manufacture, ultrapure water, not just pure water, is required because the impurities contained in the wafer-rinsing water has a great influence on the product yield. For example, any particulates, included in the impurities, will remain as evaporation residues causing stains on the wafers, thereby reducing the yield. Thus, to attain an improved yield, further reduction in the amount of impurities in the rinsing water is still required, even in the case of ultrapure water. The increasing degree of integration of semiconductor devices as a result of recent remarkable developments of the semiconductor industry has led, for example, to production of 64M DRAMS in addition to the mass production of 4M and 16M DRAMs. As a result, more strict control is being required for the permissible values (in terms of number and size) regarding the above-mentioned stains (residues) remaining after the rinsing operation. Accordingly, the standard for the quality of ultrapure water has become higher, thus making it necessary to further refine the water to an ultra-high level of purity.
However, it is extremely difficult to remove the very small amount of impurities, in particular, particulates, which are on the order to several ppb or less, out of ultrapure water, a water which has been refined to an extremely high level of purity. Experimentally, it might be possible to remove such impurities. However, in the case of equipment used on an industrial scale, that would be difficult because of the various problems involved. For example, a distillation method or the like cannot be adopted for continuous production of ultrapure water. Further, use of activated carbon as an adsorbent is also undesirable since that would involve the problem of removing the very minute particles released therefrom. Thus, from the industrial point of view, a technique to solve these problems must be realized before the above-mentioned small quantities of impurities can be removed from ultrapure water.
In connection with the inorganic particulates causing evaporation residues of ultrapure water, to which the present inventors have directed special attention, Dr. Marc Edwards and Dr. Mark M. Benjamin have proposed a method according to which colloidal substances are removed by converting them to solid oxide particulates throgh forced oxidation using ozone ("A Mechanistic Study of Ozone-Induced Particle Destabilization", JOURNAL AWWA, 1991). According to the method proposed, ozone is bubbled into pure water, thereby changing the colloidal substances in pure water to minute oxidized particles. As a result, the number of particles removed by an ultrafiltration membrane or the like is increased. The present inventors analyzed pure water which had undergone this processing and ascertained an increase in the amount of minute particles, which could be removed by membrane filtration.
The present inventors prepared ultrapure water by bubbling ozone into pure water and performing ion exchange and membrane processings thereon, and used this ultrapure water for the rinsing of semiconductors, only still to find stains on the wafer surfaces.
An object of the present invention is to make it possible to remove the impurities in ultrapure water, which impurities give rise to stains on the wafer surface. The present inventors conducted an analytical examination in order to identify the substance causing the occurrence of stains, which cannot be prevented by the above-described method, according to which colloidal substances were solidified by oxidation using ozone, and found that it was the non-ionic silica component (colloidal silica) of the colloidal substances that gave rise to the stains.
The silica component of the colloidal substances contained in ultrapure water adversely affects the performance of the semiconducter products, as pointed out, for example, in Japanese Patent Laid-Open No. 62-39769. Thus, in the conventional pure water producing apparatus shown in FIG. 9, the non-ionic particulates are removed as much as possible by membrane separation (RO or the like), and the ionic silica component is removed by an ion exchange resin through a mixed-bed ion exchange polisher. However, as shown in FIG. 7, there is no proper method of completely removing the non-ionic silica component, which is allowed to pass through the membrane, such as a reverse osmosis membrane, and through the ion exchange resin. In FIG. 7, the shadowed sections of the graph represent the concentration of the ionic silica component, and the remaining blank sections of the graph represent the concentration of the colloidal silica component.
Silica concentration in ultrapure water is generally measured in accordance with JIS-K0101. That is, the ionic silica dissolved in water is allowed to react in an acidic region with ammonium molybdate to generate molybdenum yellow, which is reduced to molybdenum blue and, by measuring the absorbance of this molybdenum blue, the silica is detected. However, as can be seen from the above description, detection of the silica is only possible when the silica exists in the water as ions. Non-ionic silica generates no molybdenum blue. Thus, to measure the non-ionic silica, it is necessary to convert it to ionic silica.
To examine the behavior of silica in water, the present inventors measured the concentrations of ionic and non-ionic silica components contained in water which has undergone the above-described process, in which colloidal substances were changed to minute solid oxide particles by forced oxidation using ozone, and the concentrations of ionic and non-ionic silica components contained in water which has undergone no such process. FIG. 8 shows the results of the measurement. As shown in the drawing, of the total silica 1, the ionic silica component 2 contained in raw water was hardly increased even by ozone bubbling, so that there was substantially no reduction in the concentration of the non-ionic silica. In the graph of FIG. 8, section 3 represents ionic silica contained in raw water which has undergone only ozone bubbling; section 4 represents ionic silica contained in water which has undergone ozone bubbling and then ultraviolet irradiation; and section 5 represents ionic silica contained in raw water which has undergone ultraviolet irradiation while undergoing ozone bubbling.
It was found through the above measurement that the non-ionic silica component in pure water could not be removed by the above-described method, according to which colloidal substances were removed by changing them to minute solid particles by forced oxidation. No attempt has been made, in the production of ultrapure water, to ionize the non-ionic silica component contained in water and to remove it by an ion exchange resin. According to the known method described in JIS-K0101 (or Japanese Patent Laid-Open No. 62-39769), the non-ionic silica component contained in water is ionized by adding sodium hydrogen carbonate to the water and by boiling the same. However, although the above-described method can be utilized for analysis and measurement purposes, it cannot be applied to industrial equipment for continuously producing ultrapure water because it requires a boiling process, etc.
The present inventors have assiduously investigated the possibility of providing a method of removing the non-ionic silica component in ultrapure water which can be applied to industrial equipment for producing ultrapure water and found, during the course of this investigation, that although the non-ionic silica component in ultrapure water could not be solidified into particulates even by ozone bubbling and, consequently, could not be removed, it could be ionized by allowing it to react with the ozone in the water under particular conditions (in terms of concentraiton and time) and by performing, at the same time, ultraviolet irradiation thereon. That is, in addition to the above examination described with reference to section 3 of FIG. 8, the present inventors dissolved ozone, in different concentrations, in water containing a non-ionic silica component to effect a reaction for a predetermined length of time and carried out ultraviolet irradiation afterwards or simultaneously, (Refer to sections 5 and 6 of FIG. 8, respectively.) and measured the concentration of the ionic silica. As a result, it was found that the ionic silica concentration had substantially increased. This processing method can be easily incorporated in a part of equipment for producing pure water or ultrapure water, with the result that an industrial apparatus for continuously producing pure water or ultrapure water can be realized.